Field of the Invention
The present invention relates to a power supply apparatus, an image forming apparatus, and a noise filter, and more particularly, to a power supply apparatus including capacitors each being connected from an AC line to a ground.
Description of the Related Art
A power supply apparatus using a commercial AC power supply as an input thereof hitherto has a filter as a device configured to suppress switching noise of an AC/DC converter or the like or terminal noise due to reverse recovery of a diode bridge. As the filter, a noise filter including a common mode choke coil, an X capacitor (across-the-line capacitor), and a Y capacitor (line bypass capacitor) is generally used. For example, a related-art power supply apparatus illustrated in FIG. 7A and FIG. 7B includes a first noise filter 210 between a commercial AC power supply 100 and a diode bridge 201 serving as a rectifying device. Further, the power supply apparatus illustrated in FIG. 7A and FIG. 7B includes a second noise filter 220 between the diode bridge 201 and an isolation transformer 203 (see, for example, Japanese Patent Application Laid-Open No. H05-2008). Terminal noise in a power supply apparatus 110 using the commercial AC power supply 100 as an input thereof includes noise caused when a field-effect transistor (hereinafter referred to as FET) 204 operates and noise caused due to reverse recovery of the diode bridge 201. Those kinds of noise are suppressed by the first noise filter 210 and the second noise filter 220. In general, Y capacitors 213, 214, 223, and 224 forming the noise filters are arranged on respective AC lines, and are connected to a frame ground 122 via a ground terminal 123 or the like formed on a substrate 121. The frame ground 122 is connected to a ground of the commercial AC power supply 100. Detailed description of FIG. 7A and FIG. 7B is made below.
However, as illustrated in FIG. 7B, there is, for example, a case in which ground patterns 127 of the Y capacitors 213 and 214 are connected to each other on the substrate 121 and the ground patterns 127 have a common impedance 128 before reaching the ground terminal 123. In FIG. 7B, a pattern from a node A between the ground patterns 127 of the Y capacitors 213 and 214 to the ground terminal 123 is the common impedance 128 of the Y capacitors 213 and 214.
A potential at the point A of the impedance 128 changes depending on noise suppressed or absorbed by one of the Y capacitors 213 and 214. Specifically, for example, noise suppressed or absorbed by the Y capacitor 213 changes the potential at the point A, and thus, even if noise is not suppressed or absorbed by the Y capacitor 214 at that time, the potential at the point A that is a ground potential for the Y capacitor 214 changes. Therefore, noise is also caused in the Y capacitor 214, and the Y capacitor 214 operates so that the potential of the AC line is constant in accordance with the potential at the point A. As a result, the potential of the AC line is fluctuated to increase the terminal noise. In other words, the effect of suppressing noise by the Y capacitors 213 and 214 cannot be fully exerted.